The present invention relates generally to magnets, and more particularly to a method for operating a superconductive magnet.
Superconducting magnets include, but are not limited to, closed and open magnet designs. Closed magnets have a single magnetic assembly with a bore in which is located the working magnetic field volume. Open magnets have two spaced-apart magnetic assemblies with generally coaxially aligned bores and a working magnetic field volume located in the open space between the magnetic assemblies. Open magnets have advantages in certain applications such as in MRI (magnetic resonance imaging) medical imaging where the open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design. A magnetic assembly contains one or more superconductive coils whose wound superconductor may embody any number of superconductive filaments. An example of a superconductor is a single niobium-tin tape filament sandwiched between upper and lower copper blankets for electrical stabilization surrounded by a paper sheath for electrical insulation.
Superconducting magnets must be cooled to below the critical temperature of the superconductor for superconductivity to occur. Known cooling techniques employ liquid cryogens (e.g., liquid helium) or cryocoolers, as is well known to those skilled in the art. Cryocoolers are more prone to temperature rises, such as a very slow rise in temperature over the lifetime of the cryocooler. Eventually, a cryocooler will be considered to fail when the temperature of the superconductor reaches its critical temperature and the magnet thereupon quenches (i.e., loses its superconductivity). It is noted that the magnet also may be intentionally quenched, by activating heaters, whenever it is desired to terminate the magnet's superconductive mode of operation.
Magnetic resonance imaging magnets have an inhomogeneity of the magnetic field in the working magnetic field volume due to manufacturing tolerances and site conditions. In many applications, the open or closed magnet must be shimmed to reduce the inhomogeneity of the magnetic field in the working magnetic field volume to within a predetermined specification. For example, an open MRI magnet whose magnetic assemblies are superconductive coil assemblies must be shimmed to reduce the inhomogeneity of the magnetic field in its working magnetic field volume, which is its imaging volume, to within a few parts per million to produce images which are sharp enough to be useful in medical diagnosis.
Known methods for shimming closed superconductive MRI magnets include active shimming and passive shimming. Active shimming typically requires a complex arrangement of superconductive shimming coils. Passive shimming typically involves the placement of carbon steel shims of calculated thickness in the bore of the closed magnet at calculated locations on the inside diameter of the superconductive coil assembly. The thickness and location of the shims are determined through use of a computer shim code, as is known to those skilled in the art, which calculates adding shims to reduce the inhomogeneity of the mapped magnetic field in the imaging volume of the closed MRI magnet. The calculated shims are added to the magnet, the magnetic field of the magnet is again mapped, and the computer shim code is again run. This process is repeated until the inhomogeneity of the measured magnetic field in the imaging volume is reduced to within a predetermined specification. The repetitive nature of the shimming process is the result of the computer shim code being only an approximation of the real magnet.
Superconductive magnets have a design current, an operating temperature, and a superconductor, wherein the superconductor has a critical temperature greater than the operating temperature. Known methods for operating superconductive magnets include the steps of ramping up the superconductive magnet to generally the design current at the operating temperature, then shimming the superconductive magnet to a desired level of homogeneity, and then using the magnet. If the desired level of homogeneity is lost (typically when ferromagnetic material is introduced near the magnet), the magnet is again shimmed. Applicants found that it was necessary, in operating superconductive magnets having less than twenty-five superconductive filaments, to reshim them whenever the operating temperature increased (such as when a faulty cryocooler was replaced with a new one of slightly-higher temperature or when a cryocooler slowly increased temperature over time) even though the operating temperature remained below the critical temperature and even though there was no change in manufacturing tolerances and/or site conditions What is desired is a method for operating a superconductive magnet having less than twenty-five superconductive filaments that does not require reshimming for such slight temperature rises.